The present disclosure relates generally to a power transmission device operable to selectively transfer torque between first and second sets of drivable wheels of a vehicle. More particularly, the present disclosure is directed to a power transmission device with a hydrostatic torque control system.
Due to increased demand for four-wheel drive vehicles, power transmission systems are more frequently being incorporated into vehicle driveline applications for transferring drive torque to the wheels. Some vehicles include a power transmission device operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. At least one known torque transfer mechanism includes a dog-type lock-up clutch that may be selectively engaged for rigidly coupling the secondary driveline to the primary driveline when the vehicle is operated in the four-wheel drive mode. When the lock-up clutch is released, drive torque is delivered only to the primary driveline and the vehicle operates in a two-wheel drive mode.
Another type of power transmission device, referred to as a transfer case, may be operable to automatically direct drive torque to the secondary wheels without any input or action on the part of a vehicle operator. When traction is lost at the primary wheels, a four-wheel drive mode is entered. Some transfer cases are equipped with an electrically-controlled clutch actuator operable to regulate the amount of drive torque transferred through a friction clutch to a secondary output shaft. The actuator typically includes an electric motor to provide an application force to the friction clutch.
While many power transfer devices are currently used in four-wheel drive vehicles, a need exists to advance the technology. For example, packaging concerns, weight and electrical power requirements of the power transmission device may make such systems cost prohibitive in some four-wheel drive applications.
A power transmission device includes a rotatable input shaft, a rotatable output shaft and a ring gear fixed for rotation with the input shaft. A carrier is fixed for rotation with the output shaft. A pump assembly includes an inner rotor supported for rotation on the carrier and an outer rotor encompassing the inner rotor. The outer rotor is in driving meshed engagement with the ring gear. The pump provides pressurized fluid to one of first and second fluid ports. A flow restrictor is moveable to selectively restrict fluid flow relative to one of the first and second ports.